NASHVILLE, Tennessee (AP) -- The disarray surrounding lethal injection in the U.S. has begun to steer states back toward methods of execution that many had long ago deemed less humane than the needle.

Tennessee jumped out front this week with a law that could essentially bring back the electric chair. Elsewhere around the country, lawmakers have been talking about reviving the firing squad and the gas chamber, methods largely abandoned a generation ago.

The reason: Lethal injection -- the primary means of execution in all 32 states with capital punishment -- is under fire as never before because of botched executions, drug shortages caused by a European-led boycott, and a flurry of lawsuits over the new chemicals that states are using instead.

The U.S. Supreme Court reinstated the death penalty in 1976, but around the country the number of executions has declined steadily since peaking in 1999. The number of states that carry the death penalty has also been declining, with six dropping it in the last eight years.

The Tennessee legislation signed into law by Republican Gov. Bill Haslam on Thursday would allow the state to use electrocution against any current or future death row inmate if lethal injection drugs become unavailable.

In truth, Tennessee never did abandon the electric chair; killers who committed their crimes before the state adopted lethal injection in 1999 have been given the choice of electrocution or the needle.

But the new law could take that choice away from the inmates and make everyone on death row subject to the electric chair.

Deborah Denno, a professor at Fordham Law School who has studied executions for more than two decades, called Tennessee's law unprecedented.

"No state has gone backward, to go back in time to a prior method of execution," she said. "For over a century, they have all moved forward."

Some attorneys warned that changing the method of execution on inmates who were originally subject to lethal injection would be unconstitutional.

Other experts noted that the legal and political attacks on lethal injection as cruel and unusual have had the unintended effect of driving states toward methods that are considered even worse.

Wyoming lawmakers are talking about changing the law to allow the firing squad, while their counterparts in Utah have proposed repealing a law that ended firing squads for prisoners convicted after 2004. Missouri law allows for the gas chamber, and politicians from both parties last year suggested rebuilding one.

During the electric-chair debate in Tennessee, one lawmaker said he would support hanging and the firing squad, too. Another legislator, Republican Rep. Dennis Powers, the bill's House sponsor, shrugged off legal and ethical concerns about the measure.

"It's not our job to judge. That's God's job to judge," Powers said. "Our job is to arrange the meeting."

Douglas Berman, a law professor at Ohio State University, said the new Tennessee law could be part of an effort to force death row inmates challenging lethal injection to back off.

"It might be the design that, 'Hey, if you fight against this hard enough and say we can't use lethal injection, fine, we'll strap your guy in the chair,'" Berman said.

Nashville defense attorney David Raybin, a former prosecutor who wrote Tennessee's death penalty statutes in 1976, warned that the switch could entangle the state in legal challenges that could prolong death row cases for many years.

Tennessee has no lethal injection drugs on hand, though officials insist they can obtain them when needed. There are 74 prisoners on the state's death row, and the next execution is scheduled for October.

The last time Tennessee used the electric chair was in 2007, when Daryl Holton, who killed his three sons and a stepdaughter with a rifle in a garage, chose electrocution.

Raybin, who witnessed the execution, called it barbaric and inhumane and complained about the "carnival-like atmosphere." The whole process, he said, "lends no dignity to the judicial system, no dignity to the victim."

The Supreme Court has never declared a method of execution unconstitutionally cruel and unusual. It upheld the firing squad in 1879, the electric chair in 1890 and lethal injection in 2008.

First used by New York State in 1890, the electric chair was employed throughout the 20th century to execute hundreds and is still an option in eight states.

The chair was considered humane when it was first introduced but has resulted in a number of horrific executions over the years.